I used to work at a factory in IL. It's actually where your Winchester Shot Gun Shells are made.
Every winter, they'd send plant department people up to the higher open spaces of the plant WHERE pigeons would gather for the winter and lay down poisoned bird seed.
Guess what else would eat the bird seed? Black birds and starlings.
Pigeons, Starlings and blackbirds will eat the same poison bird seed that pigeons do because they're wintering in the same warm(er) spaces as the pigeons.
After the poison bird seed was laid down, all the pigeons and black birds would take flight and mysteriously circle and ONE by one, they'd fall out of the sky dead. We'd have hundreds of MOSTLY pigeons dead all over the property for weeks.
I know farmers lay it down as well...to kill the starlings and black birds. Remember that Ark. is the duck hunting capital of the USA. Professional hunters are trying to attract ducks and geese, NOT red wing black birds to their fields for hunting.
So, more than likely, this is a poisoning issue. The fish and game is NOT going to release this information because they don' want animal rights people messing up their hunting seasons. So, like the factory where I worked, they're keeping it hush, hush.
It's poisoning.